1. Field of the Invention
The present invention relates to a parallax correcting mechanism for a photographic real image type viewfinder and more particularly for such viewfinder having a zoom capability.
2. Description of the Related Art
Photographic cameras equipped with an objective lens having a zoom capability have been widely used to ensure that the user can enjoy photography and easily photograph for a variety of purposes. As a recent trend, a higher magnification is demanded for this zoom functionality to meet the requirements for multi-purpose photographing.
Popular photographic cameras have usually been equipped with a direct viewfinder of virtual or real image type and the viewfinder itself has been required to have a zoom capability that is dependent on the zoom function of the photographic objective lens. Additionally, as the zooming magnification of the photographic lens has been improved, it has become more and more important for such viewfinders to have a zoom capability of correspondingly higher magnification. Such popular photographic cameras have generally been equipped with a real image type viewfinder in order to realize a photographic camera that has an increased zoom capability while sacrificing compactness.
The real image type viewfinder generally comprises an objective lens serving to form an inverted image of an object to be photographed, an erecting optical system serving to convert this inverted image to an erected image enlarged so as to be observed as a real image, and a field frame defining an image plane of this real image. Accordingly, when dust or dirt enters the viewfinder and clings to an area adjacent the field frame, such dust or dirt will be enlarged and seriously obstruct observation of the object to be photographed. To overcome this problem, the conventional real image type viewfinder comprises an objective side optical system and an ocular side optical system both contained within a common sealed casing so that dust or dirt can be prevented as perfectly as possible from entering the viewfinder.
However, increased zooming magnification of the viewfinder inevitably causes the following problems. With the classic viewfinder, both the working accuracy and the positioning accuracy of the components constituting the viewfinder optical system did not practically affect parallax and were negligible even when a sealed viewfinder unit was mounted on a camera body because the classic viewfinder was of a relatively low magnification. Accordingly, no parallax correction was necessary after the viewfinder had been mounted on the camera body. However, as the zoom magnification of the viewfinder has been increased, the working accuracy and the positioning accuracy of the optical components constituting the viewfinder have caused a significant error with respect to a theoretical parallax expected after assembly of the viewfinder. Efforts to improve the working accuracy and the positioning accuracy of the components sufficiently to eliminate the error generated during assembly of the viewfinder increases working costs of the components and therefore increases manufacturing costs of the photographic camera. This is one reason why parallax correction is necessary after assembly of the viewfinder.
So far as an Albada viewfinder is concerned, such parallax correction can be achieved by moving the field frame and/or the lenses. However, it is impossible for the real image type viewfinder to move various components such as the field frame and the prisms since viewfinders of this type have a sealed structure. Furthermore, it is also impossible to move the lenses of the objective side optical system since these lenses are adapted to be zooming-driven. For these reasons, parallax correction of the real image type viewfinder has conventionally been difficult.